The present invention relates generally to firing circuits for thyristor power conversion systems and more particularly to an improvement for controlling a DC to AC load side converter which supplies power to a synchronous machine from a polyphase alternating current source.
Many circuits and systems are known for controlling the conductivity of controlled rectifiers utilized in various types of converters for supplying electrical power to a load such as an AC motor from a polyphase alternating current (AC) source. The type of rectifier used controls, to some degree, the type of control utilized. However, by far the most common controlled rectifier used today is a thyristor of the silicon controlled rectifier type which becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to its gate electrode and which thereafter remains conductive until the anode current falls below the value required to hold the thyristor in the conductive state.
Whereas motor control systems employing thyristors have been implemented using analog control techniques, a typical example being U.S. Pat. No. 4,230,979, entitled, "Control Current Inverter And Motor Control System", issued to Paul M. Espelage et al., on Oct. 28, 1980, attention has been and is presently being directed to digital types of control techniques, examples of which are disclosed in: U.S. Pat. No. 3,601,674, entitled, "Control System For Firing SCR's In Power Conversion Apparatus", John A. Joslyn et al., which issued on Aug. 24, 1971; U.S. Pat. No. 4,263,557, entitled, "Power Converter Control", which issued to Willard B. Jarvinen on Apr. 21, 1981; and U.S. Pat. No. 4,276,505, entitled, "Microcomputer Based Control Apparatus For A Load-Commutated Inverter Synchronous Machine Drive System", which issued to Bimal K. Bose on June 30, 1981. The teachings of these patents are also intended to be incorporated herein by reference.
In any phase control system utilizing a thyristor bridge converter, whether it be an analog or digital type system, the AC terminal voltage is a prime feedback signal employed for the control of the conductivity of the various thyristors. Typically, the AC phase voltages referred to a fictitious neutral are utilized to develop line-to-line terminal voltages from which synchronizing signals are generated from pseudo-flux waveforms obtained by integrating the line-to-line voltages. In such applications, however, the zero voltage commutation notches appearing in the line-to-line voltages generate flat spots in the integrated output voltage which can occur at the zero crossings which are utilized to form a synchronizing pulse train at six times the line frequency. Where the flat spots occur at the zero crossings, the stability of the phase lock loop can be undesirably affected. In order to overcome this problem, the above cross referenced U.S. Pat. No. 4,399,395 discloses an improved technique for removing the commutation notches by summing the integrated output of the corrupted line-to-line voltages with a signal proportional to the commutating inductance multiplied by a fictitious "delta" current which is derived by taking the difference between the actual line currents in a manner well known to those skilled in the art. The resultant or composite voltages are the primary feedback control signals for synchronizing either a fixed frequency source side converter or a variable frequency load side converter or both.